Performance of a green wall (Total Value Wall™) at high greywater loading rates and Life Cycle Impact Assessment

Nature-based greywater (GW) treatment and reuse in urban areas has become an up-and-coming option. A 14.4 m² green wall system called Total Value Wall (TVW) was installed at a terraced house in Gent (Belgium) for treating GW and reusing the effluent for toilet flushing. In a previous study, the TVW was loaded at 7 L.m^-2.d^-1 and efficiently removed TSS (67%), COD (43%), BOD₅ (83%) and total coliforms (log 2), but a number of issues were reported related to nutrient leaching from the substrate, and the excessive retention time in the storage tanks. In this study results are reported from a follow-up study during which an adapted TVW was subjected to both higher hydraulic and pollutant loading rates in order to investigate the treatment capability of TVW. The design of the system, i.e. substrate contained in geotextile bags, did not sustain the higher hydraulic loading rates as excessive leakage occurred. Despite this, the higher pollutant loading rates still resulted in an acceptable effluent quality with 15 mg.L^-1 TSS (90%), 85 mg.L^-1 COD (82%), and 15 mg.L^-1 BOD₅ (95%). Ammonium, E. coli and total coliforms were removed with removal rates of 98%, 63% (0.4 log units), and 36% (0.2 log units), respectively. Finally, a life cycle assessment (LCA) was performed for the TVW with and without treating GW to analyze the environmental burden. The LCA impacts showed that replacing tap water and chemical fertilizer by GW, and the reuse of effluent, have a positive impact. However, the energy use for pumping has a major impact and should be minimized by using an efficient pump and distribution system to reduce the overall footprint.